1. Field of the Invention
This invention relates to a cathode active material suitably employed in a so-called Li-MnO.sub.2 cell (lithium-manganese dioxide cell) making use of metal lithium as anode active material.
2. Description of the Prior Art
The lithium cell making use of metal lithium as anode active material is known as being of the high energy density type because of the large capacity per unit weight of metal lithium, the cell also having a cell voltage as high as approximately 3 V because of the potential of metal lithium. Since the lithium cell can be easily reduced in size and weight while assuring a high energy density, it is increasingly used for example as a backup power supply for an IC memory.
It is known that, in this type of the cell, the selection of the cathode active material is critical and may govern cell characteristics. The Li-MnO.sub.2 cells are also known in the art in which manganese dioxide which is the cathode active material of the manganese dry cell is used as the cathode active material. These cells are inexpensive to manufacture because the manganese dioxide is readily available at low costs.
Such manganese dioxide may for example be .gamma.-MnO.sub.2 such as chemical manganese dioxide (CMD) obtained by addition of an alkali permanganate solution to a boiling neutral manganese sulfate solution, or electrolytic manganese dioxide (EMD) obtained by electrolytic oxidation from the acid bath of manganese sulfate or manganese chloride, as in the case of aforementioned manganese dry cell. These .gamma.-MnO.sub.2 forms are not desirable because they contain a large amount of water which may react with metal lithium of the anode active material to cause an electrolyte leakage or corrosion of the battery can or otherwise deteriorate the preservative character of the cell. In addition the cell capacity may be lowered due to admixture of Mn.sub.2 O.sub.3 which is not an active material. It is also known to make use of manganese dioxide obtained by heat-treating the aforementioned .gamma.-MnO.sub.2 at a temperature of the order of 150.degree. to 450.degree. C. for removing the water content. (It should be noted that the resulting manganese dioxide has undergone changes in the crystal structure by the heat-treating temperature so that it exhibits a .gamma.-phase at lower than 250.degree. C., a .gamma...beta.-phase at 250.degree. to 350.degree. C. and a .beta.-phase at higher than 350.degree. C.). Although an improvement is attained in this case, satisfactory results have not been obtained with respect to increasing the cell capacity or realizing an improved flatness of the discharge voltage.
Above all, when the lithium-manganese dioxide cell is used as a backup power supply for the IC memory, an improved flatness of the discharge voltage and an increased capacity of the cell are desired, so that a demand exists for further improving the cathode active material.